As there are soils which are more easily nitrified than others, so there are those which more readily bring forth disease than others; and it is the duty of hygiene to search further into the biological processes going on in the ground. We are still very far from the end: a boundless sea of probabilities lies before us; but that need not prevent our weighing anchor and spreading sail.
Hitherto we have allowed ourselves to be guided in hygienic practice and technics predominantly by the feelings, instincts, and what we call common sense, and have only very recently begun to establish our practice, which is, indeed, very primitive, on a scientific and experimental footing. Dirt and impurity have till now been somewhat indefinite conceptions. We use the terms whenever our innate or cultivated sense of cleanliness is unpleasantly affected; they are generally called out by impressions on our sense of smell, taste, or sight. What we call cleanliness plays an important part in daily life, in a similar manner to that which conscience, the sense of right and wrong, partly innate, partly inculcated, plays in our moral life. Just as it can be regarded as a fact that conscientious men as a rule accomplish more and better than unconscientious, so cleanly men as a rule are healthier and live longer than uncleanly men. As conscience is more or less developed in different degrees of human civilization, so also is the sense of cleanliness. Under the guidance of analogous feelings, we have instinctively and empirically found out what it is to our advantage to eat and drink, and how we should clothe ourselves, before these subjects could be dealt with scientifically.
Our established hygiene also was probably based in the first instance chiefly on the suggestions of feeling. Those who are moving in the new scientific direction should guard against considering all that is not scientifically confirmed as wrong and unfounded, but they should also not hesitate to subject established rules to a thorough scientific and experimental criticism. This will necessarily teach us that mere feeling has dictated much to us that rests upon false suppositions, and can either be omitted or must be changed. Practice or technics always precedes science. Our branches of trade and industry also began on empiric roads, and were carried on in them for thousands of years; but how greatly have they been changed, improved, and simplified, and how many new branches have arisen since we began to apply the sciences of physics, chemistry, and mechanics to them!
Hygiene, or the science of health, has a widely extended field of labor in laying a scientific foundation for our sanitary regimen. Even if in many cases it only shows that all is not as it has heretofore been supposed to be, that has its great practical value. What harm did it do to medical practice that the Vienna medical school in its day showed up the errors of the then prevailing views, and radically assailed the practice founded upon them which had been adopted for